1. Field of the Invention
This invention relates to apparatus for placing a uniform, predetermined charge onto a member having an insulating layer.
2. Description of the Prior Art
Although the corona-charging apparatus of the present invention has general applications, one preferred application is in the field of electrophotographic apparatus (herein called copiers). In copier corona-charging apparatus, a generally uniform electrostatic charge is deposited on a segment of an imaging member having a photoconductive insulating layer. The charged segment is then advanced to an exposure station where it is exposed to image-forming radiation to form a latent electrostatic image of a document to be copied. The latent image is thereafter developed and subsequently transferred to paper upon which the copied image is to appear.
Consistent, high quality reproduction can best be obtained when a uniform level of charge is applied to the imaging member by the corona-charging apparatus. The contrast value of the electrostatic latent image is related directly to the level of charge on the imaging member before exposure. If the photoconductor is not uniformly charged over the entire area, the contrast value of the latent image obtained upon exposure will vary in different areas of the imaging member, and a mottled effect will be visible on the image when developed.
The current from a corona-emitting electrode is a function of the electrode diameter and the potential applied thereto. Variations in the potential will cause relatively large changes in corona current with corresponding variations in the charging rate. The corona current is also affected by deposits of dust that may accumulate on the electrode and by variations of movement and ionized conditions of the air surrounding the electrode. Thus, minute differences in electrode diameter, slight accumulations of dust on the electrode, and variations in air current or in air pressure drastically affect the corona generating potential of the electrode, causing non-uniform electrostatic charging of the imaging member.
Conventional corona-charging apparatus employ an AC power supply coupled to the corona-emitting electrode through a series of rectifiers to obtain a high DC potential at the electrode. The need for high voltage components to rectify the AC supply current and to properly handle the total current demands of the system greatly add to the expense of the apparatus.
The need for rectification of an AC supply current applied to the corona-emitting electrode has been eliminated in known charging apparatus by locating a control electrode (known as a grid), to which has been applied a potential approximately equal to that to which the photoconductor is to be charged, between the corona-emitting electrode and the imaging member. A DC voltage is impressed upon the grid to regulate the flow of ions from the AC supplied corona-emitting electrode to the imaging member. Although this system produces a uniform charge without the need for rectifying the AC supply current, the system does so at the expense of more components, including a DC power supply for the grid.
In certain known corona-charging apparatus, such as disclosed in commonly-assigned U.S. Pat. No. 3,370,212 which issued on Feb. 20, 1968 to L. F. Frank, the need for a DC power supply for the grid has been eliminated by connecting the grid to reference potential through a rectifier and resistor. Even though there is no active power supply for the grid, there is a control voltage imposed at the grid, which voltage results from the current passing through the resistor.
While the level of charge placed on the imaging member by the Frank apparatus can be regulated to some extent by, for example, the relative movement of the corona-emitting electrode and the imaging member, such control methods are imprecise and involve complicated drive mechanisms. I have intended corona-charging apparatus for applying a potential to the insulating layer of an imaging member using only passive grid control elements in the grid circuit, wherein the applied potential is easily regulated and is extremely uniform.